This invention relates to differential amplifiers and, in particular, to transistor differential amplifiers.
A typical known transistor differential amplifier, as shown in FIG. 1 of the drawings, comprises a matched pair of transistors 1 and 2 (NPN transistors are shown). The emitter electrodes of both transistors are directly coupled to one another and are coupled to a negative power source through a common emitter resistor 3. The collector electrodes are coupled to a positive power source through resistors 4 and 5, respectively.
Input signals e.sub.1 and e.sub.2 are applied to the base electrodes of transistors 1 and 2, respectively, and antiphase or opposite-phase output signals v.sub.1 and v.sub.2 are obtained from the collector electrodes of the transistors 1 and 2. In case only one of the output signals, for example, the signal v.sub.1, is desired in practical use, the collector resistor 5 on the other output side may be omitted.
The differential amplifier of FIG. 1 has a disadvantage that a high common-mode rejection ratio (C.M.R.) is not achieved. When the input signals e.sub.1 and e.sub.2 are equal to one another in magnitude and phase, the gain of the amplifier or the common-mode gain Gc is given as follows: ##EQU1## where R.sub.3, R.sub.4 and R.sub.5 represent resistances of the resistors 3, 4 and 5, respectively. Hence, the common-mode is present in the output signals v.sub.1 and v.sub.2.
In order to achieve a high C.M.R., it is known to use a constant current source circuit 6 in place of the common emitter resistor 3, as shown in FIG. 2. It is also well known that FET transistors may be used in place of bipolar transistors in differential amplifiers. A transistor differential amplifier has been used as a first stage amplifier in a throughout direct coupled audio frequency amplifier, and also used as a driver in the throughout direct coupled amplifier.
As above described, anti-phase differential-mode signals which are equal to one another in DC level can be obtained in known differential amplifiers. But since antiphase and/or in-phase differential-mode output signals which are different to one another in DC level cannot be obtained, it was impossible to directly drive complementary symmetry circuits to operate as a push-pull amplifier by outputs of the differential amplifier.
Furthermore, since the maximum current flowing through either one transistor is restricted to twice the current at a time when no input signal is applied, due to the common emitter resistor (3, in FIG. 1) or the constant current source circuit (6, in FIG. 2), the high current operation is not possible.
It is desirable that a differential amplifier has the following features:
1. High common-mode rejection ratio (C.M.R.).
2. A circuit operative with the flow of a high current in spite of the application of the low bias current.
3. Two outputs signals which are in-phase.
4. In-phase output signals or opposite-phase output signals having different DC levels.
Another method for improving the C.M.R. of a differential amplifier is disclosed in U.S. Pat. No. 3,497,824. In FIG. 1 of the U.S. Patent, an emitter follower amplifier is coupled to a common junction in the emitter circuit of the emitter-coupled differential amplifier. Other forms of differential amplifiers utilizing four transistors are shown in FIGS. 2-4 of said patent. The C.M.R. of the circuit of FIG. 2 of the U.S. Patent appears to be higher than that of the circuit of FIG. 1, but not so high as desired.
The circuit of FIG. 2 of the U.S. Patent does not have features 3 and 4 set out above.
The circuits shown in FIGS. 3 and 4 of the U.S. patent do not realize a high C.M.R., because the common-mode signals are equivalently grounded at resistors 82 and 83.
Neither of the circuits of FIGS. 3 and 4 of the U.S. patent has the feature 2 set out above. The emitter current of transistors 20 and 21 flows through the resistor 82, and the emitter current of transistors 65 and 66 also flows through the resistor 82. Therefore, it is required that the current corresponding to the peak current which flows through transistors 20, 21, 65, and 66 must be applied to flow from sources 86 and 87 through resistor 83, diode 80 and resistor 82. This means that the use of a greater electric power source is necessary.
The circuit of FIGS. 3 and 4 of the U.S. patent do not have the aforesaid features 3 and 4.
Accordingly, it is an object of this invention to provide a transistor differential amplifier wherein differential-mode output signals which are different to one another in DC level can be obtained.
It is another object of this invention to provide a transistor differential amplifier which is operative without restriction of the current flowing therethrough.
It is a further object of this invention to provide a transistor differential amplifier wherein a high common-mode rejection ratio is achieved.
A specific object of this invention is to provide a transistor differential amplifier which can directly drive a complementary symmetry circuit to operate as a push-pull amplifier.
Further objects and features of this invention will be understood from the following description of preferred embodiments of this invention referring to the annexed drawings.